1. Field of the Invention
The invention relates to an image forming apparatus for use in a copying machine, a printer, a plotter, a facsimile machine, or any apparatus having a printing function.
2. Description of Related Art
There has been hitherto proposed an image forming apparatus in which an image is formed using an aperture electrode unit having plural openings (hereinafter referred to as "apertures"). In this image forming apparatus, a voltage is selectively applied to the aperture electrode unit in accordance with image data to control toner particles to selectively pass through the apertures to form an image on a supporter (image forming medium) with the toner particles which pass through the image apertures of the aperture electrode unit. This type of image forming apparatus is disclosed in the specification of U.S. Pat. No. 3,689,935.
The aperture electrode unit includes an insulating flat plate, a reference electrode which is continuously formed on one side surface of the flat plate, plural control electrodes which are formed on the other surface of the flat plate so as to be electrically insulated from one another, and at least a row of apertures which are provided in correspondence with the respective control electrodes so as to penetrate through the insulating flat plate, the reference electrode and the control electrodes.
The image forming apparatus includes the aperture electrode unit as described above, a voltage supply means for selectively applying a potential across the control electrodes and the reference electrode of the aperture electrode unit on the basis of the image data, a toner supply means for supplying charged toner particles so that the flow of the toner particles passing through the apertures is modulated in accordance with the potential applied to the aperture electrode unit, and a positioning means for positioning the supporter in a particle-flowing path relatively to the aperture electrode unit.
Further, U.S. Pat. Nos. 4,743,926, 4,755,837, 4,780,733 and 4,814,796 disclose an image forming apparatus in which the aperture electrode unit is disposed so that the control electrodes thereof face the supporter side and the reference electrode thereof faces the toner supply side.
On the other hand, U.S. Pat. No. 4,912,489 discloses an image forming apparatus in which the aperture electrode unit is disposed so that the reference electrode thereof faces the supporter side and the control electrodes thereof face the toner supply side. As disclosed in the latter U.S. patent, the image forming apparatus can reduce the voltage to be applied to the control electrodes at an off time to about a quarter of that of the image forming apparatus as disclosed in the former U.S. patents.
Here, the term "off time" means a time when no toner particle is attached onto the supporter, that is, a time when a blank portion of an image is formed. Conversely, the term "on time" means a time when a toner image is formed on the supporter. In the conventional image forming apparatus as described above, the insulating sheet of the aperture electrode unit is formed of an extremely thin insulating member such as a polyimide film having a 25 .mu.m or less thickness. Such an extremely thin film is liable to wrinkle or be scratched, and its contact state with a toner carry member at the toner supply side is not stable because of its low rigidity so that no stable recording operation can be performed.
In order to solve this problem, it may be proposed that the insulating sheet be designed to have a thickness of 50 .mu.m or more with the control electrodes disposed to face the toner carry member at the toner supply side to improve the rigidity of the sheet without deteriorating the controllability of the electric field. However, it has become clear that in some cases print density is lower when a control voltage is applied to this type of aperture electrode unit to perform a printing operation. Through studies and consideration of this phenomenon, it has been proved that the electric field force used to pass the toner through the insulating sheet becomes weaker as the thickness of the insulating sheet increases.